1. Field of the Invention
This invention relates to a method and an apparatus for encoding and communicating an audio signal in a digital mobile telephone system or a like system.
2. Description of the Related Art
Conventionally, in a signal communication system such as a digital mobile telephone system, in order to reduce the amount of data of an audio signal to be transmitted, an encoding transmission apparatus compresses the audio signal by encoding and transmits the encoded audio signal, and a reception decoding apparatus decompresses the received audio signal by decoding.
Normally, in conversation of human beings, a sound-present period is not continuous, but a sound-present period and a sound-absent period appear alternately. However, in a signal communication system of the type called non-silence compression type, the encoding transmission apparatus successively encodes and transmits an audio signal without making a distinction between a sound-present period and a sound-absent period, and the reception decoding apparatus successively receives and decodes the encoded data.
However, since it is wasteful to communicate also in a sound-absent period in this manner, in a signal communication system of the type called silence compression type, encoded data only in a sound-present period are transmitted, whereas encoded data in a sound-absent period are not transmitted, thereby further reducing the amount of data to be transmitted.
In such a signal communication system of the silence compression type as described above, since the encoding transmission apparatus intermittently transmits only encoded data in sound-present periods, a processing operation in a sound-absent period and a background noise inserting operation of the reception decoding apparatus must be matched with a processing operation of the encoding transmission apparatus. Therefore, in order to prevent deterioration in quality of an audio signal, various processes are executed.
Various conventional examples which relate to such a signal communication system as described above are disclosed in Japanese Patent Laid-Open Application No. 334197/95, Japanese Patent Laid-Open Application No. 314098/94, Japanese Patent Laid-Open Application No. 357735/92, Japanese Patent Laid-Open Application No. 109840/91, Japanese Patent Laid-Open Application No. 83399/94, and so forth.
The encoding transmission apparatus disclosed in Japanese Patent Laid-Open Application No. 334197/95 as a conventional example is described below with reference to FIG. 1. FIG. 1 is a block diagram showing a signal encoding unit of the encoding transmission apparatus.
The signal encoding unit 101 includes an input wiring 102, to which an audio signal is inputted. A sound-presence/absence discrimination unit 103 and an encoding processing unit 104 are connected in parallel to each other to the input wiring 102, and the output of the sound-presence/absence discrimination unit 103 is connected also to the encoding processing unit 104.
A parameter working unit 105 is connected to the encoding processing unit 104, and the sound-presence/absence discrimination unit 103 is connected also to the parameter working unit 105. An error correction unit 106 is connected to the parameter working unit 105, and the error correction unit 106 is connected to an output wiring 107.
The sound-presence/absence discrimination unit 103 mentioned above discriminates an audio signal between a sound-present period and a sound-absent period, and the encoding processing unit 104 encodes the audio signal in a sound-present period in accordance with the CELP system and outputs a speech parameter. However, in a sound-absent period, the encoding processing unit 104 encodes the audio signal at a first initial interral and after each subsequent fixed interral of the sound-absent period in a similar manner as in a sound-present period and outputs a speech parameter similarly as in a sound-present period.
The parameter working unit 105 mentioned above processes a speech parameter encoded by the encoding processing unit 104 based on the result of discrimination of the sound-presence/absence discrimination unit 103 mentioned above. More particularly, the parameter working unit 105 invalidates a long term predicted delay (LAG) which relies on a previous state of the audio parameter, processes a long term predicted gain into a minimum quantized value and outputs the quantized value.
The error correction unit 106 for correcting an error of encoded data is connected to the parameter working unit 105. The error correction unit 106 is connected to the output wiring 107 for outputting encoded data. Since the error correction unit 106 encodes an audio parameter, the encoded data is outputted as a compressed audio signal from the output wiring 107.
Since the audio signal encoded and compressed by the signal encoding unit 101 as described above is transmitted from the encoding transmission apparatus to the reception decoding apparatus, the reception decoding apparatus decodes and decompresses the received audio signal.
When the encoded data in a sound-absent period are to be decoded, a long term predicted signal which makes use of a correlation with a previous signal is invalidated, and encoded data sent thereto at fixed intervals are successively interpolated during a time within which no encoded data are sent thereto so that the interpolated encoded data are decoded to obtain sound that is not incongruous.
Meanwhile, the reception decoding apparatus disclosed in Japanese Patent Laid-Open Application No. 314098/94 discriminates, after it receives and decodes an audio signal, a sound-absent period from a succession of minimum values in quantized values of the power (sound volume) of the audio signal, and suppresses, in a sound-absent period, the power of the reproduced sound to reduce noise.
In the signal communication system disclosed in Japanese Patent Laid-Open Application No. 357735192, in order to cope with a level (sound volume) variation of background noise during a sound-absent period, an encoding transmission apparatus transmits a noise level at fixed intervals within the sound-absent period. A reception decoding apparatus inserts background noise into the sound-absent period based on the received noise level so that natural sound may be reproduced.
In the signal communication system disclosed in Japanese Patent Laid-Open Application No. 109840/91, an encoding transmission apparatus transmits, when a level variation of background noise is detected in a sound-absent period, noise level data and an identifier that represents whether that encoded data are abandoned. A reception decoding apparatus distinguishes abandonment of encoded data and a sound-absent period from each other based on the identifier.
By this construction, the encoding transmission apparatus and the reception decoding apparatus can operate equivalently in a sound-present period and a sound-absent period. Consequently, background noise in a sound-absent period can be regenerated with a high fidelity, and also missing of encoded data in a sound-present period can be prevented.
In the reception decoding apparatus disclosed in Japanese Patent Laid-Open Application No. 83399/94, in a sound-absent period, a code vector of zero or of a small magnitude is inputted to a synthesizer filter which synthesizes speech with the code vector. The level of the decoded sound is reduced smoothly to prevent production of click noise or like noise at an instant of changing over from a sound-present period to a sound-absent period.
Either such a signal communication system of the silence compression type in which no communication is performed in a sound-absent period or another signal communication system of the non-silence compression type wherein communication is performed also in a sound-absent period as described above, can be applied to a signal communication system of an ATM (Asynchronous Transfer Mode) system or an SDH (Synchronous Digital Hierarchy) system.
However, it is not easy to connect a signal communication system of the silence compression type having a variable bit rate and another signal communication system of the non-silence compression type having a fixed bit rate to each other.
In short, in a signal communication system of the silence compression type, in a non-transmission period of encoded data in a sound-absent period, decoding processing for encoded data is not performed but interpolation processing of background noise is performed as in the conventional example disclosed in Japanese Patent Laid-Open Application No. 334197/95. In the meantime, in another signal communication system of the non-silence compression type, encoding and decoding of an audio signal are performed in all periods irrespective of whether sound is present or absent.
If encoded data are directly transmitted from an encoding transmission apparatus of the silence compression type to a reception decoding apparatus of the non-silence compression type, then since the encoding transmission apparatus of the silence compression type does not transmit encoded data in a sound-absent period, the signal processing apparatus of the non-silence compression type cannot perform decoding processing in a sound-absent period.
In order to prevent this, when encoded data are to be transmitted from the encoding transmission apparatus of the silence compression type to the reception decoding apparatus of the non-silence compression type, at a connection location between the signal communication systems of the silence compression type and the non-silence compression type, it is necessary to decode the encoded data of the silence compression type into an audio signal and then encode the audio signal into data of the non-silence compression type.
In other words, encoding and decoding are performed once for each of the silence compression type and the non-silence compression type, and consequently, data are deteriorated significantly by the repetition of processing. Further, since an encoding transmission apparatus and a reception decoding apparatus of different ones of the silence compression type and the non-silence compression type cannot be connected directly to each other, the entire system is complicated in structure.
The signal communication systems of the documents mentioned above are all directed to improvement in sound quality in a sound-absent period and cannot allow mutual connection between a signal communication system of the silence compression type of a variable bit rate based on compression of silence and another signal communication system of the non-silence compression type that processes encoded data of a fixed bit rate.